Patent Document 1 discloses, as a method for producing 3-methyl-2-thiophenecarboxylic acid, a method for producing 3-methyl-2-thiophenecarboxylic acid which comprises reacting a 3,4-dihalobutan-2-one with a thioglycolic acid in the presence of a base. However, there are problems with industrial application of this method such as the use of malodorous thioglycolic acid.
Further, Patent Document 2 discloses a method for producing 3-methyl-2-thiophenecarboxylic acid which comprises chlorinating 3-methylthiophene to prepare a Grignard reagent of 2-chloro-3-methylthiophene, and reacting the Grignard reagent with carbon dioxide. However, this method requires two reaction steps of the chlorination reaction and the Grignard reaction from 3-methylthiophene, and a dichloro-form is formed as a by-product by the first chlorination reaction. Accordingly, this method should be further improved as an industrial method for producing 3-methyl-2-thiophenecarboxylic acid.
As a more simple method for producing 3-methyl-2-thiophenecarboxylic acid, a method of directly carboxylating 3-methylthiophene may be mentioned. For example, Non-Patent Document 1 discloses a method for directly carboxylating 3-methylthiophene in the presence of a Lewis acid. However, there are problems with industrial application since this method requires use of a Lewis acid such as pyrophoric ethyl aluminum dichloride under pressure conditions. Further, this method is insufficient for industrial application also in view of the yield and the selectivity.
A method of directly carboxylating a thiophene using chlorosulfonyl isocyanate is disclosed in Patent Documents 3 and 4.
However, the method disclosed in Patent Document 3 does not involve a problem of regioselectivity (selectivity for 3-methyl-2-thiophenecarboxylic acid and 4-methyl-2-thiophenecarboxylic acid) in carboxylation of 3-methylthiophene since thiophene is used as a raw material. Further, the yield of the obtainable 2-thiophenecarboxylic acid is not so high as from about 50% to about 75%.
The method disclosed in Patent Document 4 is a method of reacting a thiophene with chlorosulfonyl isocyanate and reacting the reaction product with an amine to obtain a N-sulfamoyl-2-thiophenecarboxamide, not a method for producing a thiophenecarboxylic acid. Further, it failed to specifically disclose an example relating to the reaction of a thiophene with chlorosulfonyl isocyanate.